Thermal cracking has been widely used for obtaining propylene and aromatic hydrocarbons from hydrocarbon feedstock, but by its very nature thermal decomposition requires severe reaction conditions, and a common by-product is methane, which is difficult to use as petrochemical feedstock. Moreover, the product yields of propylene and other olefins and benzene, toluene and other aromatic hydrocarbons as percentages of the decomposition product are generally limited, and the yield structure is not sufficiently flexible among other problems.
Patent Document 1 discloses a method wherein silver is carried on crystalline aluminosilicate zeolite in order to improve selectivity for lower olefins. Although propylene yield is improved by this method, the yield of aromatic hydrocarbons is poor. Also, in methods for catalytic conversion of hydrocarbons using zeolite, coke accumulates on the catalyst and must be frequently removed by combustion to regenerate the catalyst, but the problem is that in the case of the acid zeolite described above, repeated regeneration operations cause permanent degradation of catalyst activity. This occurs because coke combustion generates steam that hydrolyzes the zeolite, causing aluminum to be released from the zeolite crystals and eliminating protons that are active sites in the catalyst. This poses a serious problem that must be solved if proton-type zeolites are to be used in these kinds of reactions.
Patent Document 2 discloses a proton-free zeolite catalyst along with a method for using this catalyst to convert hydrocarbon feedstock into ethylene, propylene and monocyclic aromatic hydrocarbons. The catalyst used in this method has the advantage of being resistant to regeneration degradation, but is still vulnerable to the problem of coking deterioration. Moreover, because the aforementioned paraffin conversion reaction is an endothermic reaction, a large amount of heat must be supplied to the reaction vessel. As a result, this method requires a complex and expensive reaction system.
An effective means of increasing the flexibility of the propylene/aromatic hydrocarbon yield structure is to obtain these components by separate processes. However, the reaction technologies that have been used for each in the past need to be improved in order to obtain both components efficiently and stably.
For the propylene production step, methods of catalytic conversion from hydrocarbon feedstock containing olefins using catalysts containing zeolite have been adopted, and there are many reports of methods for producing propylene from hydrocarbon feedstock containing olefins by catalytic conversion using catalysts containing zeolite. However, efficient, long-term and stable production of propylene from hydrocarbon feedstock containing olefins by catalytic conversion using a catalyst containing zeolite is difficult for the following reasons.
Propylene is an intermediate in reactions for converting olefins into aromatic hydrocarbons in the presence of a zeolite catalyst, and is converted into aromatic hydrocarbons by the subsequent reaction. Consequently, when attempting to produce propylene by catalytic conversion of hydrocarbon feedstock containing olefins using a catalyst containing zeolite, the catalyst activity and reaction conditions need to be strictly controlled in order to obtain the desired product with a high yield. That is, if the catalyst is too active or the contact time is too long, the resulting propylene will be converted to aromatic hydrocarbons by the subsequent reaction. Conversely, if the catalyst is not active enough or the contact time is too short, the propylene yield will be poor. Because olefins are highly reactive, however, deposition of coke on the surface of the catalyst is likely to occur during catalytic conversion of the hydrocarbon feedstock containing olefins using the catalyst containing zeolite. In continuous conversion reactions, the catalyst deteriorates due to coking (coking deterioration), and catalytic activity quickly declines. Regeneration operations are required as described above in order to reactivate the catalyst, but after repeated regeneration operations catalytic activity can no longer be adequately restored.
As discussed above, coking is particularly likely in catalytic conversion reactions of the hydrocarbon feedstock containing olefins using catalysts containing zeolite, and regeneration degradation is also extremely likely because of the consequent need for frequent regeneration operations.
Patent Document 3 discloses a method for converting C4-12 olefins into ethylene and propylene using a proton-free ZSM-5 zeolite containing an IB group metal and having an SiO2/Al2O3 ratio of from 200 to 5000. When a zeolite-containing catalyst is used to selectively convert C4-12 olefins to propylene, olefins with about 4 to 8 carbon atoms are produced as reaction products in addition to ethylene and propylene. This is because the raw material olefins are dimerized and decomposed by the catalyst, resulting in an olefin composition similar to the equilibrium composition under the reaction conditions. Consequently, in order to efficiently convert the raw material olefins into propylene, the C4+ olefins in the reaction product need to be efficiently recycled back to the reaction container by a simple method, and converted to propylene.
Patent Document 3 describes a method for removing the heavy fraction with a boiling point at or above that of the C8 aromatic hydrocarbons from the reaction product and recycling the C4-8 olefins back into the reaction vessel, but this method requires multiple separators to obtain the raw material for recycling, thereby complicating the equipment and operations, so there is a demand for simpler methods, but so far it has not been possible to achieve both efficiency (equipment, operating costs and yield) and stable production. Patent Document 3 also makes no mention of the effect on coking deterioration of the diolefin concentration in the hydrocarbon feedstock. The higher the diolefin compound concentration in the hydrocarbon feedstock, the more activity deteriorates due to coke production. Removal of diolefins from the feedstock is highly unpractical on an industrial scale because it requires that the feedstock be purified by pretreatment such as separation by distillation, partial hydrogenation or the like.
For the aromatic hydrocarbon producing step, however, many methods are known for producing aromatic hydrocarbons using zeolite catalysts. As in the propylene producing step, the biggest problems with methods of producing aromatic hydrocarbons by catalytic cyclization using a zeolite catalyst are controlling coking deterioration during the reaction and controlling regeneration (permanent) degradation that occurs when the coke on the deteriorated catalyst is removed by combustion to regenerate the catalyst.
Many proposals have been made in recent years for preventing both kinds of deterioration. Specifically, Patent Document 4 reports that coke precipitation during the reaction and permanent degradation due to dealumination during catalyst regeneration can be simultaneously controlled by using a high-silica zeolite catalyst of a specific particle size that exhibits a specific surface acid site/total acid site ratio and amounts of pyridine adsorption before and after steam treatment, or in other words specific changes in acid site behavior. In this method, however, the preferred method of synthesizing the zeolite is one using a seed slurry, which has low productivity, and moreover the stable production range of ZSM-5 zeolite is narrow, limiting the SiO2/Al2O3 ratio so that the primary particles are likely to be relatively large. According to Patent Document 4, both coking deterioration and regeneration degradation are controlled, but because the zeolite particle size is relatively large, it does not appear that coking deterioration is adequately controlled. A zeolite with a smaller primary particle size would be preferable, but according to Patent Document 4 this results in greater accumulation of coke during the reaction, and more rapid regeneration (permanent) degradation. These are serious obstacles to the industrial manufacture of aromatic hydrocarbons.
An example using a proton-free zeolite is given in Patent Document 2. The catalyst used in this method is effective at resisting regeneration degradation as described above, but the problem of coking deterioration remains. Consequently, coking deterioration is likely when the hydrocarbon feedstock contains large amounts of olefins. Moreover, this document makes no mentioned of the effect of the particle size of the zeolite used in the catalytic cyclization reaction.
[Patent Document 1] Japanese Patent Application Laid-open No. H02-184638
[Patent Document 2] WO 1996/013331, pamphlet
[Patent Document 3] WO 2000/010948, pamphlet
[Patent Document 4] Japanese Patent Application Laid-open No. H10-052646